I'm assuming this has something to do with the update to 19.4 that somehow messed up the Lemmy Crawler.
awesome-lemmy-instances seems to have a similar problem.
I don't think @lodion's information is correct because I remember seeing lemmy.ml in the instance filter list just a while ago.
Also: What does any of this have to do with the moderation of lemmy.ml? I also don't like their moderation but does that mean the instance and their communities shouldn't be visible on the Lemmy Explorer? Who should decide which instance should or should not be included on the list? If you want to avoid the instance like I do you can just use the filter to remove lemmy.ml communities on the community browser of the Lemmy Explorer.